I have a basic question:
What is the difference between "Run Apple Script (async in background)" and "Run Apple Script (blocking)"?
I've seen each used in the various presets / triggers shared by others, but don't quite understand the different use cases.
Thanks!
It's complicated 
Apple Scripts that present UI will often need the blocking version, otherwise the UI (e.g. dialogues) won't show up. However this will block any other code execution in BTT.
The async ones run in the background but can't show UI. They should be used whenever possible because they will be more performant.